Honda HT3810 Owner's Manual page 46

6.Thread the plug in by hand to prevent cross threading.
7. After a new spark plug has been seated by hand, it should be tightened
l/2 turn with a wrench to compress its washer.
If a used plug is being reinstalled, it should only require 118 to l/4 turn
after being seated.
CAUTION:
l
The spark plug must be securely tightened. An improperly tightened plug
can become very hot and possibly damage the engine.
l
Never use a spark plug with an improper heat range.